This invention relates to a magneto-optic recording medium that is directly overwritable by optical modulation, enabling new information to be written directly over old information, and to a manufacturing method for this magneto-optic recording medium, and apparatus for recording on and reproducing from such magneto-optic recording medium.
First Prior Art Example
In FIG. 99, (a) is an oblique view of the main parts of a prior-art magneto-optic read-write device as shown, for example, in Preprints of the 34th Joint Congress of Applied Physics, Spring 1987, 28 P-Z L-3; (b) is a sectional view illustrating optical reading and writing of the recording medium; and (c) is a plot of the laser power variations for writing information in areas on the recording medium. In these drawings, 1 is a magneto-optic recording medium comprising a glass or plastic substrate 2, a first magnetic layer 3, and a second magnetic layer 4. An exchange coupling force acts between the first and second magnetic layers 3 and 4, tending to align their magnetization in the same direction. A laser beam LB is focused by an objective lens 5 onto a spot 6 on tile information medium 1. The numeral 7 indicates areas In which the direction of magnetization in the first magnetic layer 3 is upward in FIG. 99 (b), this indicating the recording of binary "1" data. An initializing magnet 9 generates a magnetic field of substantially 5000 oersteds to initialize the second magnetic layer 4. A bias magnet 8 disposed facing the objective lens 5 with the information medium 1 in between generates a magnetic field of substantially 200 to 600 oersteds. In FIG. 99 (c) laser power is shown on the vertical axis and areas are indicated on the horizontal axis. The laser power is modulated to record the information "1" in the region R1 and the information "0" in the region R0. The dash-dot line in FIG. 99 (a) separates new data (DN) on the left from old data (DO) on the right.
The operation will be explained next. The recording medium 1 is rotated in the direction of the arrows in FIG. 99 (a) and (b) by a support and driving mechanism not shown in the drawing. The first magnetic layer 3 has the same properties as the recording layer in the media used in general magneto-optic disks comprising, for example, Tb.sub.21 Fe.sub.79, and here too it functions as a reading and writing layer. The second magnetic layer 4, called the auxiliary layer, comprises Gd.sub.24 Tb.sub.3 Fe.sub.73, for example, and provides the overwrite function, enabling new information to be written over old information in real time. The Curie temperatures Tc1 and Tc2 of the first and second magnetic layers 3 and 4, their room-temperature coercivities Hc1 and Hc2, and their room-temperature exchange coupling strengths Hw1 and Hw2 satisfy the following relations: EQU Tc1&lt;Tc2 EQU Hc1-Hw1&gt;Hc2+Hw2
First the reading of information recorded in the first magnetic layer 3 (the recording layer) will be explained. As shown in FIG. 99 (b), the first magnetic layer 3 is magnetized in the up direction to represent a "1" and in the down direction to represent a "0." When this information is read, the first magnetic layer 3 is illuminated by the beam spot 6, and the magnetic orientation of the first magnetic layer 3 in the beam spot 6 is transformed by the well-known optical Kerr effect to optical Information, in which form it is detected. FIG. 100 indicates the temperature changes in the magnetic layers in the spot caused by the laser beam power, with A corresponding to the intensity of the laser beam that illuminates the recording medium 1 during reading. At this intensity the maximum temperature increase in the first and second magnetic layers 3 and 4 in the beam spot 6 does not attain the Curie temperatures Tc1 and Tc2 of these layers, so the illumination in the beam spot does not erase the direction of magnetization; that is, it does not erase the recorded information.
Next the overwriting operation will be explained. The initializing magnet 9 in FIG. 99 generates a magnetic field of intensity Hini in the direction of the arrow b (up) in the drawing. The field Hini is related to the coercivity and exchange coupling strength of the first and second magnetic layers 3 and 4 as follows: EQU Hc1-Hw1&gt;Hini&gt;Hc2+Hw2
As a result, when the information medium 1 revolves in the direction of the arrow a in FIG. 99 (b), those parts of the second magnetic layer 4 that pass over the initializing magnet 9 are uniformly magnetized in the up direction, regardless of the magnetic alignment of the first magnetic layer 3. The first magnetic layer 3 itself is not affected at room temperature by the magnetic field of the initializing magnet or by the exchange coupling force exerted by the second magnetic layer 4, so it remains in its previous state.
To write a "1," which means to magnetize the first magnetic layer 3 in the up direction, the laser beam is modulated to the intensity B in FIG. 100. The temperature in the beam spot 6 then rises above the Curie temperature Tc1 of the first magnetic layer 3, but does not reach the Curie temperature Tc2 of the second magnetic layer 4. Consequently, the first magnetic layer 3 loses its magnetization, while the second magnetic layer 4 retains the upward magnetic alignment given by the initializing magnet 9. As the disk turns and the area leaves the illumination of the beam spot 6, when the temperature of the first magnetic layer 3 falls below its Curie temperature Tc1, the magnetic alignment of the second magnetic layer 4 is transferred to the first magnetlc layer 3, so that the first magnetic layer 3 becomes magnetized in the up direction, corresponding to a "1."
To record a "0," which means to magnetize the first magnetic layer 3 in the down direction, the laser beam is modulated to the intensity C in FIG. 100. The temperature in the beam spot 6 then rises above both the Curie temperature Tc1 of the first magnetic layer 3 and the Curie temperature Tc2 of the second magnetic layer 4. Consequently, the first and second magnetic layers 3 and 4 both lose their magnetization. As the disk turns and the area leaves the illumination of the beam spot 6, when the temperature of the second magnetic layer 4 falls below its Curie temperature Tc2, the second magnetic layer 4 is magnetized in the down direction by the weak magnetic field applied in the direction of the arrow c (down) in FIG. 99 by the bias magnet 9. Moreover, when the temperature of the first magnetlc layer 3 falls below its Curie temperature Tc1, the magnetic alignment of the second magnetic layer 4 is transferred to the first magnetic layer 3, so that the first magnetic layer 3 becomes magnetized in the down direction, corresponding to a "0."
By the above overwriting operations, new information can be written over old information in real time by modulating the laser beam power between the values B and C in FIG. 100 according to the binary codes "0" and "1" of the new information.
Second Prior Art Example
Another example of prior art magneto-optic recording medium is shown in FIGS. 101 and 102. This magneto-optic recording medium is described in Japanese Patent Application Kokai Publication No. 268103/1988, as Embodiment 1 in this publication. This magnetic recording medium 101 comprises a first magnetlc thin film 100, a second magnetic thin film 200, a third magnetic thin film 300, a fourth magnetic thin film 400, a transparent substrate 500, a dielectric film 600, and a protective film 700. Reference numeral 900 denotes an interface magnetic wall. The magnetic thin films 100 to 400 are formed of transition metal (TM)--rare earth metal (RE) alloy magnetic materials. With this medium, recording is made under application of an external magnetic filed Hex, by heating the medium either to a first temperature T1 not lower than Curie temperature Tc1 of the first magnetic thin film 100, or to a second temperature T2 at which the orientation of the sublattice magnetization of the second magnetic thin film 200 can be reversed. The Curie temperatures are related as follows: EQU Tc1&lt;Tc2 (0-1) EQU Tc4&lt;Tc2, Tc3 (0-2) EQU Tc4.ltoreq.Tc1 (0-3)
At room temperature, the state of the magnetization is either in the state A or the state C. When the temperature is increased to T1, the first magnetic thin film 100 loses its magnetization (state E in FIG. 102). When the temperature falls below Tc1, the sublattice magnetization orientation of the first magnetic thin film 100 is aligned with the sublattice magnetization orientation of the second magnetic thin film 200. When the temperature fails to room temperature, the state A is assumed. Thus, a section or bit cell in which "0" has been recorded is formed.
When the temperature is increased to T2, the first and the second magnetic thin films 100 and 200 lose their magnetization. The second magnetic thin film 200 will then be magnetized by the external magnetic field Hex, and hence its sublattice magnetization orientation is reversed (state F in FIG. 102. When the temperature falls to the vicinity of Tc1, the sublattice magnetization orientation of the first magnetic thin film 100 is aligned with the sublattice magnetization orientation of the second magnetic thin film 200. This transfer of the sublattice magnetization orientation is similar to that which takes place when the temperature is increased to Tc1. However, the sublattice magnetization orientation of the second magnetic thin film 200 is opposite and the transfer must take place by the exchange-coupling alone (without the aid of the external magnetic field). The following relation therefore must be satisfied. EQU .sigma.w1&gt;2.multidot..vertline.Ms1.vertline..multidot.h1.multidot.Hex(0-5)
where h1 represents the thickness of the first magnetic thin film,
Msi represents the magnetization,
Hci represents the coercivity, and
.sigma. w1 represents the energy density of the interface magnetic wall between the first and the second magnetic thin films.
The external magnetic field Hex cannot therefore made high. It is described in this publication that Hex is not more than about 1 kilo-oersteds. An interface magnetic wall 900 is created at the fourth magnetic thin film 400 because the sublattice magnetization orientations of the second and the third magnetic thin films 200 and 300 are opposite to each other (state G in FIG. 102. When the temperature falls further from this state G to room temperature TR, the state C is assumed provided that the following relationships are satisfied: EQU .sigma.w2-2.multidot.Ms3.multidot.h3.multidot.Hex EQU &lt;2.multidot.Ms319 h3.multidot.Hc3 (0-11) EQU .sigma.w2-.sigma.w1-2.multidot.Ms2.multidot.h2.multidot.Hex EQU &gt;2.multidot.Ms2.multidot.h2.multidot.Hc2 (0-12)
where .sigma. w2 represents the energy of the interface magnetic wall between the second and the third magnetic thin films 200 and 300. Thus, a section or bit cell in which "1" has been recorded is formed.
The above mentioned publication also shows another medium, as Embodiment 2, of which the process of magnetization is shown in FIG. 103. FIG. 104 is a temperature characteristics diagram of the magnetization and coercivity of the second magnetic thin film 200. In the FIG. 103, the arrows of the broken line denote RE sublattice magnetization. The magnetic thin films are composed as shown in Table P1.
TABLE P1 ______________________________________ Magneti- Curie zation Coercivity Com- Temp. (emu (kilo- Thickness Film position (.degree.C.) cc.sup.-1) oersteds) (angstroms) ______________________________________ First TbFeCo 150 100 12 500 Second GdTbFeCo 210 100 1 300 Third TbFeCo 150 150 7 500 Fourth TbFe 130 -- -- 100 ______________________________________
The second magnetic thin film 200 has the temperature characteristics of the magnetization and coercivity shown in FIG. 104. The external magnetic field Hex is so set as to satisfy EQU Hc2&lt;Hex
In the example described, the external magnetic field Hex is 1 kilo-oersteds. The recording operation is similar to that described above. But the initialization of the second magnetic thin film 200 is achieved by setting the external magnetic field to be higher than the coercivity of the second magnetic thin film 200 so as to satisfy the same condition as the recording medium shown in FIG. 102.
Third Prior Art Example
A further example of recording medium in the prior art is shown in Japanese Patent Application Kokai Publication No. 241051/1989. In this prior art, four magnetlc layers are provided, and overwriting is achieved without resorting to the external magnetic field. The overwriting in the prior art is shown in FIG. 105. The fourth magnetic layer is premagnetized so that its sublattice magnetization orientation is upward, for example. At room temperature, the sublattice magnetization orientations of the second and the third magnetic layers are identical with the sublattice magnetization orientation of the fourth magnetic layer (FIG. 105 at (a)).
When the recording medium is heated above Tc1, the first magnetic layer loses its magnetization. When it cools below Tc1, the sublattice magnetization orientation of the first magnetic layer is aligned with the sublattice magnetization orientation of the second magnetic layer (FIG. 105 at (g)), and the first magnetic layer is magnetized upward (FIG. 105 at (f)). In this way, recording which results in upward sublattice magnetization orientation in the first magnetic layer is achieved.
When the recording medium is heated above TH, and close to Tc2, the first and the third magnetic layers lose their magnetization. The exchange-coupling from the fourth magnetic layer does not act on the second magnetic layer, and because of opposing magnetic field, the second magnetic layer is magnetized so that its sublattice magnetization orientation is opposite to the sublattice magnetization orientation of the fourth magnetic layer (FIG. 105 at (e)). When the temperature is decreased below Tc1, the sublattice magnetization orientation of the first magnetic layer is aligned with the sublattice magnetization orientation of the second magnetic layer, to be downward (FIG. 105 at (i)). When the temperature is returned to room temperature, the sublattice magnetization orientation of the second magnetic layer is returned to the initial state (this process is called initialization) by exchange-coupling with the third magnetic layer (FIG. 105 at (h)). In this way, recording which results in the downward sublattice magnetization orientation in the first magnetlc layer is achieved.
Problems Associated with the Prior Art Examples
The first-mentioned prior-art magneto-optic recording medium has a problem that an initializing magnet with a strong magnetic field is required and the overall structure of the read-write apparatus is complex and large in size.
A problem associated with the medium of Embodiment 1 of the Japanese Patent Application Kokai Publication No. 68103/1988 is that the external magnetic field Hex must be small so that the initialization of the second magnetic layer 200 is restrained, as will be seen from the condition (0-12) for the transition from the state G to the state C. However, if the external magnetic field Hex is set small, it is difficult to reverse the magnetization orientation of the second magnetic layer 200 into direction of the external magnetic field Hex when the medium is heated to T2 for High writing, and it may fail to realize the state F. Moreover, even if the conditions are so set as to satisfy the relationship (0-11), the sublattice magnetization orientation of the third magnetic thin film 300 may be reversed when the thermo-magnetlc recording medium 101 moves out of the region where the external magnetic field is applied, thereby causing failure in overwriting.
A problem associated with Embodiment 2 of the Japanese Patent Application Kokai Publication No. 268103/1988 is that the initialization of the second magnetic layer 200 utilizes the external magnetic field Hex so the external magnetic field Hex must be fairly large. Then, the transfer of the sublattice magnetization orientation of the second magnetic layer to the first magnetic layer 100 at about Tc1, i.e., the transition from the state F to the state G is difficult to occur, thereby causing failure in overwriting. Moreover, it is difficult to realize the second magnetic layer having the coercivity as set forth in Table P1, and even if it is realized, writing is difficult.
A problem associated with the medium described in Japanese Patent Application Kokai Publication No. 241051/1989 is that the writing into the second magnetic layer is made employing the compensation point recording system. Accordingly, the second magnetic layer must have a compensation temperature above room temperature and below the medium temperature during High writing. Therefore, stray magnetic field does not act, and recording characteristics are poor. For instance, in digital recording signals are not obtained at all, and overwriting was difficult. In addition, the exchange-coupling functions between the first magnetic layer and the second magnetic layer even below TL, and initialization of the second magnetic layer is not completely achieved, and overwriting may not be achieved.